SQL में समय के बीच औसत समय प्राप्त करें

SELECT  AVG(period)
FROM    (
        SELECT  TIME_TO_SEC(TIMEDIFF(@date_posted, date_posted)) AS period,
                @date_posted := date_posted
        FROM    (
                SELECT @date_posted := NULL
                ) vars,
                messages
        ORDER BY
                date_posted
        ) q